One type of particle accelerator includes an evacuated conduit or beam pipe for guiding a particle beam from an accelerator to an experimental set-up. The beam exits the pipe through a thin window of metal foil or plastic film. Rupture of the window produces a rapid inrush of air which can cause major damage to the accelerator equipment. To avoid such damage, most accelerators are equipped with a fast closing valve triggered by a pressure rise detector. However, such fast closing valves have had interior bores which were not symmetrical about the axis of the beam pipe, which results in disturbance of the particle beam. A fast closing valve which provided a valve bore that was symmetrical about the axis of the beam pipe during normal operation, and which enabled very rapid and tight closure of the beam pipe to prevent the inrush of air, would be of considerable value.